Pulsed flashlamps, and in particular Xe filled flashlamps, are used in a variety of applications, including to pump various gas or other laser devices, in various photo, copying, optical detection and optical ranging applications, in cosmetology and in various dermatology and other medical applications. Such lamps normally operate at comparatively high peak voltage, current, and light intensity/power. In order to achieve such high values, power supplies or drives for such lamps typically employ a storage capacitor, which is charged between lamp flashes or pulses, in series with an inductor and some type of switch. Examples of switches used in the past have included thyristors, which once turned on, generally remain on until the capacitor has fully discharged, and transistors. Circuits, such as disclosed in U.S. Pat. No. 4,524,289, which are a modified version of the more standard circuits indicated above, have also been used for driving flashlamps, the primary advantage of such circuits being that they require a smaller capacitor for a given flashlamp having particular voltage and current requirements. U.S. Pat. No. 4,275,335 teaches a flash lamp drive circuit which detects flash lamp current or voltage to control capacitor discharge to maintain substantially constant light intensity.
However, none of the prior art circuits have the capability of producing quickly changing programmable pulse shapes for the flashlamp output, none of these circuits provide protection for circuit components during switch turn-on transitions, something which is generally required for high powered lamp applications, and none of the current circuits are capable of maintaining constant power output from the lamp, and thus constant lamp intensity, when there are fluctuations in lamp impedance, such fluctuations occurring, and sometimes being substantial, as a result of changes in lamp temperature, and as a result of other factors.
Further, with the possible exception of the “335” patent, in none of these circuits is it feasible to produce flashlamp pulses of longer than several milliseconds, the latter problem resulting from the fact that the size of the capacitor utilized increases substantially linearly with pulse width and becomes prohibitively large for most applications beyond a few milliseconds. The size of the required capacitor for a given output is also increased by the relatively low efficiency in capacitor utilization in most of these prior art circuits, such circuits generally utilizing only 20-50% of the energy stored in the capacitor. However, there are applications, particularly medical applications, where the shape of the optical pulses is important in order to achieve a desired therapeutic effect, and in particular to achieve such effect without damage to areas of the patient's body not being treated. For example, in optical dermatology, it may be desirable to rapidly heat a target chromophore to a selected temperature, and to then reduce applied energy so as to maintain the chromophore at the desired temperature. There are also applications where pulses well in excess of a few milliseconds, for example on the order of several hundred milliseconds, may be desirable. The advantages of such long pulses in performing various optical medical procedures, including optical dermatology, is discussed in co-pending application Ser. No. 09/769,960, filed Jan. 25, 2001 and entitled METHOD AND APPARATUS FOR MEDICAL TREATMENT UTILIZING LONG DURATION ELECTROMAGNETIC RADIATION. Flashlamps are one potential source of optical radiation in such applications.
Finally, more efficient utilization of energy stored in the capacitor, which permits the use of smaller capacitors carrying lesser charge, is desirable in all flashlamp applications since it reduces the size, weight and cost of the lamp drive circuitry and enhances the safety of such drive circuits by reducing shock risks. An efficient drive circuit for flashlamps which permits pulses in excess of several milliseconds to be generated without requiring an excessively large capacitor and/or fast, programmable control of pulse shape is therefore desirable.
Another problem with flashlamps is that, in order to avoid premature failure of the lamp, it is desirable that discharge first be established in a low current density simmer mode prior to transfer to an arc mode. This is generally accomplished by triggering to initiate breakdown in the lamp with a triggering circuit, maintaining discharge with a low current DC simmer source and then providing the main current discharge for arc mode from completely separate circuitry. This duplication of components increases the size, weight and cost of flashlamp drive circuits; however, circuitry for permitting sharing of components for at least some of these functions does not currently exist.